went right into the ground. everything in its path was blown into a sheet of flame. >> initially, ntsb investigators are baffled what could have caused a failure of this magnitude. >> this is all that's left of american airlines flight 191, as far as it got on its way to los angeles. >> they sift through the wreckage and look to the plane's two flight recorders for clues. within days of the crash, it began to zero in on a part called the pylon. it has a 13-inch crack they believe was caused by fatigue prior to the crash, which maintenance inspections had missed. what exactly is a pylon? aviation consultant john cox showed us at evergreen international, an airplane maintenance and storage facility outside tucson, arizona. >> you can see the pylon assembly, on which the engine rests on the pylon and it's attached to the wing.